1. Field of the Invention
The present invention relates to a method and apparatus for enhancing data transmission efficiency in a wireless communications system, and more particularly to a method and apparatus capable of decreasing delay of a NACK status report, so as to enhance the data transmission efficiency.
2. Description of the Prior Art
To enhance transmission efficiency and reliability, a traditional wireless communications system comprises an Automatic Retransmission Request (ARQ) function, and a sliding window can be used for ARQ operations. There are two types of the sliding window, Push Type and Pull Type. The Push Type window is advanced only when a packet corresponding to the lower edge of the window is received successfully or is informed to be discarded. When a packet with SN (sequence number) outside the Push Type window is received, the packet is discarded. Therefore, the Push Type window is generally used in a receiver, such as an AM RLC entity in UMTS (Universal Mobile Telecommunications System), to provide ARQ function as specified in Radio Link Control (RLC) protocol specification, 3GPP TS 25.322 V7.0.0, developed by the 3rd Generation partnership Project (3GPP).
In comparison, the Pull Type window is advanced when a packet with an SN outside the window is received and the SN is set to be the updated upper edge of the Pull Type window. If any missing SN in the window is moved below the lower edge of the Pull Type window, i.e., outside the Pull Type window, due to the window advancement, the missing SN is neglected. Therefore, the Pull Type window is generally used in a receiver, where no ARQ function is needed, such as a reordering entity in a High Speed Downlink Packet Access (HSDPA) system, to support in-sequence delivery, as specified in Medium Access Control (MAC) specification, 3GPP TS 25.321 V7.0.0, developed by the 3rd Generation partnership Project (3GPP).
In addition, for both the Push Type and the Pull Type windows, the window sizes are kept constant unless they are reconfigured to new values. For an entity, only one type of sliding window is used.
In a wireless communications system, such as Long Term Evolution (LTE) or System Architecture Evolution (SAE) systems, which supports PS transportation only, the system architecture can be simpler than a system, such as UMTS, which supports both circuit-switched (CS) and PS transportation. In such a simplified system, Radio Link Control (RLC) and Media Access Control (MAC) layers can be combined into one layer. Since only one type of sliding window is used for an entity, the transmission efficiency will be decreased.
Firstly, the RLC entity uses Push Type window for performing ARQ function. When an SN is found missing (e.g., a later SN is received before the missing one), the RLC entity reports a negative acknowledgement (NACK) in a status report to request the peer entity for a retransmission. Besides, the MAC entity can retransmit packets through Hybrid Automatic Repeat Request (HARQ), which triggers retransmission according to positive/negative acknowledgement signals (ACK/NACK) transmitted from an HARQ process in a receiving end. The HARQ process combines unsuccessfully decoded data to generate correct packets, so that transmission efficiency can be increased. However, due to such property, a packet may arrive the receiver before another packet transmitted before the packet. Therefore, while the RLC entity transmits a status report to request the transmitter for retransmission of a missing packet, the MAC entity may get the missing packet through HARQ processes. Thus, unnecessary retransmission is performed and radio resource is wasted.
Therefore, a mechanism is required for the RLC entity to effectively detect whether an HARQ PDU (Protocol Data Unit) is lost or is under a retransmission procedure, so as to timely request for retransmission. A straightforward solution is to use a timer mechanism. When a missing SN is detected, a timer is started. The NACK status report will not be sent out before the timer expires. This can avoid pre-matured NACK status report and the unnecessary retransmission if duration of the timer is configured properly. However, HARQ is an asynchronous ARQ technology, meaning that time of completion of packet retransmission in HARQ is varied due to different transmission environment or conditions, so that proper timer duration is not easy to be configured. The timer duration need be configured long enough to cover the worst case of all the conditions that may happen to the HARQ processes. Therefore, a NACK status report may be delayed because of the timer duration configuration. This will deteriorate the data transmission efficiency.